Motors often have thermal protection, the bigger, more powerful and more expensive they are, the more likely they are protected.
Sometimes often tripping an overcurrent or thermal device will lower the point it trips at. When the motor turns itself off, put your hand on it, it ought to be pretty hot if itís tripping thermal protection.
I know not scientific, but if itís running cool, then maybe we are chasing a red herring.
DC motors may or may not be thermally protected, sort of goes back to if itís a big expensive motor or a small cheaper one.
However since itís been running for four years, I donít think we can blame design or installation
, or at least they ought to be pretty far down the list.
If it was sprayed with salt water
, then the motor itself is more suspect Iíd think, they donít like being treated like that.